282 report— 1846. 



canal. The diverging appendage, sometimes single and anchylosed to the 

 arch (lepidosiren) ; sometimes single and detached like a long, narrow bone 

 (some muraenoids) ; more commonly consists of two bones (23, 24), which 

 extend outwards, downwards, and backwards from the pleurapophysis (20) ; 

 but the more constant and better ossified bone of the two, no. 24, articulates 

 posteriorly with the succeeding pleurapophysis (2s) and combines its move- 

 ments with those of its own arch, just as the diverging appendages of one 

 thoracic haemal arch in the bird associate the movements of that arch with 

 those of the next in succession (as in fig. 15, pi, a, pi). The hasmapophyses 

 here, as at the opposite end of the body, begin so far to dissociate themselves 

 from the pleurapophyses as to articulate also directly with the centrum (13) 

 as well as with the pleurapophyses. I regard this as a very interesting ap- 

 proximation to that condition of the typical vertebra which is illustrated by 

 the diagram (fig. 14), and which is seen in nature in the caudal vertebrae of 

 the crocodiles, enaliosaurs and menopome (fig. 28, H). 



From the foregoing analysis it appears, then, that in osseous fishes the 

 endoskeletal bones of the head are arranged, like those of the trunk, in seg- 

 ments ; that these are four in number, and that they closely conform to the 

 character of the typical vertebra. 



Thus we have four centrums and neural arches : viz. 



N 1. Epencephalic arch (figs. 1 and 5, 1, 2, 3, 4) ; 



N 11. Mesencephalic arch (figs. 2 and 5, 5, 6, 7, s) ; 



N in. Prosencephalic arch (figs. 3 and 5, 9, 10, n, 12) ; 



N iv. Rhinencephalic arch (figs. 4 and 5, 13, 14, 15). 

 As a collective name for the sum of these immoveably articulated arches 

 would be as convenient as the anatomist finds the names 'sacrum' and 'cara- 

 pace,' applied to similarly consolidated portions of vertebral segments in the 

 pelvic and abdominal regions of certain air-breathing vertebrates, that of 

 ' cranium ' may well be retained for the neural arches of the skull : but it 

 should be understood to signify, in all animals, the bones 1 to 15 inclusive ; 

 whereas it has, hitherto, been applied variably in different species ; some- 

 times including sense-capsules and facial bones, intercalated to expand the 

 walls of the cavity for a large brain ; and more frequently excluding true 

 cranial bones, those of the rhinencephalic arch, for example, which encompass 

 as essential a part of the encephalic chamber, as the sacral vertebrae do of the 

 neural canal at the opposite end of the vertebral axis ; although in both in- 

 stances the extremities of the neural axis may have been withdrawn, in the 

 course of its concentrative change and movement, from their original seat. 



The haemal arches indicated by the arrows in fig. 5, the heads marking 

 the point of junction or crown, are, — 



H 1. Scapular arch (50-52) ; 



H 11. Hyoidean arch (38-43) ; 



H in. Mandibular arch (28-32) ; 



H iv. Maxillary arch (20-22). 

 The diverging appendages of the haemal arches are, — 



1. The Pectoral (54-57); 



2. The Branchiostegal (44) ; 



3. The Opercular (34-37) ; 



4. The Pterygoid (23-24). 



The bones or parts of the splanchno-skeleton which are intercalated with 

 or attached to the arches of the true vertebral segments, are, — 

 The Petrosal (16) or ear-capsule, with the otolites, 16"; 

 The Sclerotal (17) or eye-capsule ; 

 The Turbinal (19) or nose-capsule ; 

 The Branchial arches ; 



